Thin film, p-type and n-type highly conductive materials are used as contact layers in many semiconductor structures, including photovoltaic cells, particle detectors, X-ray and electromagnetic radiation detectors that convert incoming radiation to electrical output signals.
As shown in FIG. 1, such devices 2 typically include a layer of semiconductor detecting material 4 that detects the incoming particle or electromagnetic radiation 6, and emits an electrical signal 8 in response to such detection. A front (or radiation entry) surface of the detecting material is at least partially covered by a layer of thin film, preferably high conductivity material 10 to which a first electrical lead 12 is coupled. Commonly, device 2 includes a thin layer of protective transparent substrate material such as glass 14 that overlies this contact layer 10. A back surface of material 4 is at least partially covered by a layer of high conductivity material 16 to which a second electrical lead 18 is coupled. In some devices, the lower surface of back contact layer 16 is made reflective 20 to cause incoming radiation to pass through the detection layer twice, effectively increasing the device signal output. In such applications, back contact layer 16 must be thin so as not to attenuate incoming radiation enroute to the reflective region 20. Electrical contact between the device 2 and the outside world is via contact layers 10 and 16 using leads 12 and 18. The present invention is directed toward the production of an improved thin film material suitable for front contact layer 10 and, where a thin layer is required, for back contact layer 16.
Incoming radiation 6 (e.g., light, IR, X-ray, etc.) must pass at least partially through front contact layer 10 (as well as the transparent layer 14) before entering the detection layer. Therefore, to reduce attenuation, the front contact layer's thickness 22 should be minimal, preferably 200 .ANG. or less for photovoltaic cells, and 800 .ANG. or less for electromagnetic radiation detectors. Further, because electrical signals from the device 2 must pass through the upper contact layer before reaching lead 12, layer 10 should be highly conductive electrically, to minimize signal loss. Because electrical signals also pass through lower contact surface 16 before reaching lead 18;, layer 16 should also be highly conductive electrically to minimize electrical losses. However layer 16 need not be thin unless a reflective surface 20 is utilized by the device 2.
It has been surprisingly difficult in the prior art to produce highly conductive deposited thin film material for use in semiconductors. In the prior art, contact layer materials are either a heavily doped amorphous material, or a heavily doped microcrystalline semiconductor material. Semiconductor devices including such materials are not optimally designed because the radiated losses and contact layer resistance is excessive. As a result, such devices tend to be inefficient and require a relatively large amount of incoming radiation to produce a useful electrical output signal.
The prior art's use of amorphous material for contact layers, for example as the p-layer and/or n-layer in a p-i-n structure, has drawbacks because by nature amorphous materials store energy that can dissipate over time. During dissipation, the energy promotes chemical reactions that create instability and degradation in any semiconductor material using the material.
In addition, it is well known that light can cause degradation in amorphous silicon materials, and thus amorphous photovoltaic devices experience conversion efficiency degradation relatively rapidly. Further, semiconductor dopants tend not to modulate conductivity as efficiently in amorphous materials as in crystalline structures, which results in more resistive contact layers.
The other prior art approach to manufacturing a thin film contact layer avoids amorphous materials, and instead uses a fine-grained microcrystalline material, such as the material depicted for layer 10 in FIG. 1. Using this approach, the crystals are created during a deposition growth (e.g., plasma enhanced chemical vapor deposition). Post-deposition, the crystals may be subjected to an annealing process to increase the grain size and improve electrical conductivity. Often, however, annealing cannot be accomplished because the temperatures required to increase grain size (and thus improve electrical conductivity) exceed about 950.degree. C. Frequently such high temperatures cannot be tolerated by the other materials in the device, e.g., the substrate, the detector layer. For example, prior art detectors on inexpensive glass substrates do not employ such annealed contact layers because of the excessive heat that would be required.
Large grain size, where it can be achieved in contact layers, would promote improved conductivity by minimizing the number of grain boundaries, which tend to "soak up" semiconductor dopants and carriers, rendering them less effective and making the contact layer material less conductive. Generally a large grain size is associated with a large volume fraction of the crystalline material.
Unfortunately prior art deposition-created crystals are essentially isotropic microcrystals, growing during deposition no larger laterally than the film thickness. Thus as depicted schematically in FIG. 1, contact layer 10 comprises a layer of many small sized microcrystals 24, whose lateral dimensions are no larger than the contact layer's thickness 22. Further, because the crystals 24 have such small grain size, there are many grain boundaries 26 that absorb dopant as well as carriers, hindering electrical conductivity. Although larger microcrystals 24 could be grown by increasing the film thickness 22, thereby improving electrical conductivity by decreasing the number of grain boundaries 26, the increased thickness would degrade optical transmission due to increased absorption losses. Although for ease of illustration, the crystals 24 are depicted as identical cubes in FIG. 1, in reality the essentially isotropic crystals need not necessarily be identical in size and shape to one another.
Generally, such deposition produced microcrystalline silicon contact layers (doped p-type or doped n-type) have conductivities in the range of 10.sup.-3 to 10 S/cm, but only for thicknesses exceeding 800 .ANG.. As film thickness is decreased below about 400 .ANG., conductivity falls by many orders of magnitude. This conductivity loss coincides with a decrease in Raman signal strength at 520 cm.sup.-1, which corresponds to the volume fraction of Si in the crystalline phase, and coincides with an increase in the Raman signal at 480 cm.sup.-1, which corresponds to volume fraction of Si in the amorphous phase.
In summary, there is a need for an ultra-thin contact material that is highly conductive electrically. Preferably such material should be stable, easily doped and highly responsive to doping, and capable of production using a low temperature semiconductor process. The present invention discloses such a material, and a low temperature method for producing the material.